1. Field of the Invention
The present invention relates generally to electronic device assemblies wherein one electronic component is mechanically and electrically bonded to another electronic component and, more particularly, to assemblies comprising a copper-polyimide "thin film" electronic component structure and a multilayer ceramic substrate electronic component structure with an organic spacer between the components to align the contact pads of each component, which structures are mechanically and electrically bonded together by solder reflow and to a method for making the assembly without defective solder connections and increased reliability and cost-effectiveness of the manufacturing process.
2. Description of Related Art
In an electronic assembly such as a multi-chip module (MCM), a matrix of electronic devices, typically chips, are generally mounted on a multi-layer ceramic (MLC) carrier. One of the main functions of such an MLC carrier is to interconnect the chips or other electronic devices mounted to the carrier ("top-to-top" connections) and to interconnect such chips to the input-output or I/O connections of the carrier board ("top-to-bottom" connections).
For a variety of performance as well as manufacturing reasons, including cost and yield concerns, it is desirable to limit the number of MLC layers. One solution of the current art is to use "thin film electronic components," generally made of layers of polyimide, polymeric material, or other organic material having a low dielectric constant with the layers having copper wiring and interconnecting vias. Typically, the thin film is secured to one of the planar surfaces of the MLC carrier and appropriate interconnections are made between the opposing, "mating" surfaces of the thin film and the MLC carrier. A typical thin film structure, like a MLC, contains a number of interconnections using vias, contact pads and connecting conductor straps. Typically, both surfaces of the thin film structure contain contact pads for connecting the thin film package to the MLC component on one side of the thin film structure and/or chips on the other side of the structure.
The use of thin films also enhances the ability to repair certain defects in the resulting MLC carrier. Furthermore, the polyimide material and copper lines and vias of a thin film interconnect generally produce better electrical performance than the typical MLC carrier. As such, thin film technology has been a critical part of high-performance interconnect carriers for almost all MCMs.
The prior art processes of achieving active pad connections with thin film components have drawbacks and disadvantages however. In particular, the thin film structure is typically formed on a sacrificial carrier, typically glass according to prior art techniques, and the thin film is released from the carrier in a free-standing form by using suitable laser techniques. The thin film is held after release in its free-standing form by a ring or frame which engages the edges of the thin film. The free-standing thin film is then laminated to an MLC carrier or module with its pads aligned with the pads of the MLC. The pad connections between the thin film and the MLC carrier are achieved typically by gold, thermo-compression bonding.
Many of the drawbacks and disadvantages of this approach relate to the fact that the thin film is released from its sacrificial carrier in a free-standing form before lamination joining with the MLC. When the thin film is thus released, its internal stress generally will cause it to shrink, often by over 0.2% depending on the number of levels of the thin film structure. The shrinkage of the thin film makes it difficult to align the pads and other electrical connections of the thin film to the corresponding MLC carrier. Such shrinkage, and even distortion, are exacerbated if the thin film structure is released from its sacrificial carrier too soon in the manufacturing process. The solution of holding the thin film around its edges with a ring or frame after release has the additional disadvantage of reducing the active thin film structure area.
In another application semiconductor chips are attached to laminate substrates, such as printed circuit boards, rather than to MLC modules. A consistent obstacle in attaching chips to laminate substrates is the mismatch of the thermal coefficient of expansion, or TCE, between the chips and the laminated boards or substrates. This problem is especially acute in the case of direct chip attach. Direct chip attach, or DCA, is a growing trend in the microelectronics industry for many applications. Direct chip attach involves attaching semiconductors directly to a laminate substrate such as a printed circuit board or card.
Regardless of the electronic components being joined, the mismatch of TCE between the two components generates shear stress on the electrical connections between the components and such shear stresses result in fatigue of the connections over the life of the product, making those connections prone to failure. In addition, the difference in the TCE applies bending forces to the components at their outer edges and, therefore, may cause the components to crack.
One attempted solution to alleviate the problems caused by the difference in TCE is to inject an underfill epoxy between the components after the solder is joined. The underfill also imparts increased rigidity to resist bending moments which would otherwise be applied to the component. Unfortunately, the foregoing use of underfill is unworkable for joining two components which are large (typically greater than 25 mm) due to inability of the underfill to flow under such large areas.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for bonding electronic components together, particularly a thin film component to a MLC using solder bumps and reflow, without causing the component to be damaged or distorted or to lose their registration with the corresponding pads to which they are to be connected.
It is another object of the present invention to improve the quality and reliability of electrical connections made between the electronic components being joined particularly a thin film component and a MLC using solder bumps and reflow.
An additional object is to provide electronic component assemblies which are easily and reliably made and have a long operating life and particularly an assembly comprising a thin film electronic component bonded to a MLC using solder bumps and reflow.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.